[futures.async] Avoid note within a sentence.

[jensmaurer]

Partially addresses #4228

After this is merged, check page break in [futures.task.members].

[tkoeppe]

What about this other note -- would it also work better outside the list?

[jensmaurer]

Maybe. I'm trying to limit the size of the changes this late in the process.

[zygoloid]

We could further limit the size of the changes by turning these bullets into proper sentences. (There's already a period before the first note.) Eg:

Synchronization: The following apply regardless of the provided policy argument:

• The invocation of async synchronizes with the invocation of f.
  [Note:...]
• The completion of the function f is sequenced before the shared state is made ready.
  [Note:...]

For master we could be a bit more aggressive:

Synchronization: The invocation of async synchronizes with the invocation of f. The completion of the function f is sequenced before the shared state is made ready.
[Note: These apply regardless of the provided policy argument, and even if the corresponding future object is moved to another thread. However, f might not be called at all, so its completion might never happen.]

[tkoeppe]

I like both parts of this.

[jensmaurer]

This is now the minimal fix against C++20. Please check.

[jensmaurer]

The larger fix for C++23 is in #4244.